1. Field of the Invention
The present invention relates to a transfer belt device, a method of assembling the transfer belt device, and an image forming apparatus that includes the transfer belt device.
2. Description of the Related Art
A direct transfer system is well known as a transfer system for a typical image in a color image forming apparatus. In the direct transfer system, toner images of different colors which are respectively formed on a plurality of photosensitive drums are transferred, by directly superimposing in a sequence, onto a sheet that is conveyed by a transfer belt (transfer conveying belt). Due to this, a color image is formed. Furthermore, an indirect transfer system is used as another transfer system. In the indirect transfer system, the toner images of different colors, formed similarly as mentioned earlier, are transferred, by sequentially superimposing in the sequence, onto a transfer belt (intermediate transfer belt). Due to this, the color image is formed. The color image is bulk transferred onto the sheet.
The transfer belt used in the direct transfer system or the indirect transfer system is wound around a plurality of rollers including a tension roller. By using a biasing member such as a spring, the tension roller is biased and a predetermined tensile force is applied to the transfer belt.
In recent years, along with an enhancement in miniaturization of the image forming apparatus, miniaturization of the transfer belt is also called for. As disclosed in Japanese Patent Application Laid-open No. 2002-258629 or Japanese Patent Application Laid-open No. 2007-72337, the biasing member such as the spring is positioned on an inner side, which has become a dead space, of the transfer belt to miniaturize an existing transfer belt.
If a spring having a high spring constant is used as the biasing member of the tension roller, a biasing force easily fluctuates due to variations in the dimensional tolerance of components and positioning accuracy errors. For example, if the biasing force of a pair of springs that biases both ends of the tension roller fluctuates due to variations in the dimensional tolerance of the components and the positioning accuracy errors and a balance between the springs disrupts, a running stability of the transfer belt is likely to reduce. Due to this, problems such as deterioration of the transfer belt and degradation of the quality of an image that is formed on the sheet occur.
To overcome the problems, it is desirable to use a spring having a low spring constant as the biasing member of the tension roller. However, to obtain a predetermined biasing force by using the spring having the low spring constant, it is necessary to use a spring that is marginally long.
In a structure, which is mentioned in any one of the technologies mentioned earlier, in which the spring is positioned in a direction that is orthogonal to the rollers, if the long spring is used, for avoiding an interference of the spring with the rollers, the spring needs to be positioned by shifting to a vertical direction such that the spring intersects the rollers. Thus, in such a method of positioning the spring, changing a design such as further thinning and reducing a flat space, which is formed in between the transfer belt, is difficult.
As disclosed in Japanese Patent Application Laid-open No. 2000-259053, if the tension roller is connected to one end of an L-shaped lever member that is turnably supported around a shaft that is parallel to a shaft of the tension roller and if the tension roller is pressed on the transfer belt by causing the lever member to turn by using a tension spring that is arranged inside the transfer belt, a space is obtained in the vertical direction. Thus, further thinning and reducing the flat space, which is formed in between the transfer belt, is difficult.